Angle resolved photoemission from organic semiconductors: orbital imaging beyond the molecular orbital interpretation

M Dauth,P Puschnig,A Schöll,M Wiessner,V Feyer,F Reinert,S Kümmel

doi:10.1088/1367-2630/16/10/103005

Abstract

Fascinating pictures that can be interpreted as showing molecular orbitals have been obtained with various imaging techniques. Among these, angle resolved photoemission spectroscopy (ARPES) has emerged as a particularly powerful method. Orbital images have been used to underline the physical credibility of the molecular orbital concept. However, from the theory of the photoemission process it is evident that imaging experiments do not show molecular orbitals, but Dyson orbitals. The latter are not eigenstates of a single-particle Hamiltonian and thus do not fit into the usual simple interpretation of electronic structure in terms of molecular orbitals. In a combined theoretical and experimental study we thus check whether a Dyson-orbital and a molecular-orbital based interpretation of ARPES lead to differences that are relevant on the experimentally observable scale. We discuss a scheme that allows for approximately calculating Dyson orbitals with moderate computational effort. Electronic relaxation is taken into account explicitly. The comparison reveals that while molecular orbitals are frequently good approximations to Dyson orbitals, a detailed understanding of photoemission intensities may require one to go beyond the molecular orbital picture. In particular we clearly observe signatures of the Dyson-orbital character for an adsorbed semiconductor molecule in ARPES spectra when these are recorded over a larger momentum range than in earlier experiments.

Highlights

Motivated by Angle resolved photoemission spectroscopy (ARPES) experiments that can be interpreted as revealing orbital densities, we reviewed the theory of photoemission in which the Dyson orbitals, defined as the overlaps between N- and (N − 1)-particle states, emerge as the relevant quantities
When Dyson orbitals are a priori approximated by single molecular orbitals the quality of the so predicted ARPES intensities can depend sensitively on which electronic structure method is used to calculate the molecular orbitals
ARPES experiments were done for the model organic semiconductors PTCDA and NTCDA covering a larger range of electron momenta than covered by previous experiments, and using variable light polarization

Summary

Angle resolved photoemission from molecules

The imaging of molecular orbitals has attracted great attention in recent years in rather different areas of physics [1,2,3,4,5,6,7,8,9,10]. The question of what is visualized in molecular orbital (density) imaging experiments has been discussed from several perspectives [8, 13, 14] and the power of the molecular orbital interpretation of ARPES experiments has been impressively demonstrated. A decisive aspect of our approach is that it explicitly takes into account electronic relaxation effects, i.e. the changes in electronic structure that occur when one electron leaves the system The latter effect is typically neglected in the molecular orbital interpretation of photoemission data. We show that imaging experiments resolve the differences between molecular orbitals and Dyson orbitals This demonstrates that even for organic semiconductor molecules for which the molecular orbital picture is frequently employed, a detailed understanding of ARPES experiments can require one to go beyond the molecular orbital point of view

Photoemission and the Dyson orbitals

Approximate Dyson orbitals from DFT

Photoemission from the N2 molecule

Photoemission from organic semiconductor molecules

Conclusions